Intimidate looks at the current colors of a creature that has it. Normally, Halo Hunter can’t be blocked except by artifact creatures and/or black creatures. If it’s turned white, then it can’t be blocked except by artifact creatures and/or white creatures.

10/1/2009

If an attacking creature has intimidate, what colors it is matters only as the defending player declares blockers. Once it’s blocked, changing its colors won’t change that.

10/1/2009

Halo Hunter’s triggered ability isn’t optional. When it enters the battlefield, if you’re the only player that controls an Angel, you’ll have to target an Angel you control.